Brake warning light on dash

The light turns on when there is UNEVEN pressure. If the fluid level is constant, its probably not from a leak. When you say brakes dont work very well, do you mean none of the wheels will lock up ? Front only ? Rear only? I would look at master not being bled properly, was it bench bled? Did they install the calipers with bleeders pointing up? You can try bleeding again, start at RR, LR, RF, LF in that order. Tap on each caliper with screw driver handle to get bubbles that cling to caliper walls to break free. Pump 3 times SLOWLY and hold so you dont create air bubbles by agitating fluid. I that doesnt work, you most likely have bad master. Look at master with cover off and SLOWLY have someone depress pedal, look for air bubbles coming into master, which indicates bad bench bleed or most likely bad master.
SSBC masters dont have low level sensor and e brake switch being bad would make light stay on and brakes would probably work. 1996 mini van master has nothing to do with A body disk conversion....